Metal-ion catalysis in alkaline ethanolysis of 2-pyridyl thionobenzoate: Effects of modification of electrophilic center from C=O to C=S

Abstract

Pseudo-first-order rate constants (kobsd) have been measured spectrophotometrically for the nucleophilic substitution reactions of 2-pyridyl thionobenzoate (5b) with alkali-metal ethoxides (EtOM, M+ = Li +, Na+, K+, and 18-crown-6-ether complexed K+) in anhydrous ethanol at 25.0 ± 0.1 oC. The plots of kobsd vs. [EtOM]o curve upward regardless of the nature of the M+ ions, while those of kobsd/[EtO -]eq vs. [EtO-]eq are linear with a positive intercept. Dissection of kobsd into kEtO- and kEtOM (i.e., the second-order rate constants for the reactions with the dissociated EtO- and ion-paired EtOM, respectively) has revealed that the ion-paired EtOM is more reactive than the dissociated EtO-, and M+ ions catalyze the reactions in the order K+ Na + Li+ 18C6-complexed K+. The plot of log k EtOM vs. 1/rStokes results in an excellent linear correlation, indicating that the reactions are catalyzed by the solvated M + ions but not by the bare M+ ions. The reactions of 5b with EtOM have been concluded to proceed through a six-membered cyclic TS, in which the solvated M+ ions increase the electrophilicity of the reaction center and the nucleofugality of the leaving group.